Metallic workpieces with a monocrystalline structure or structures are used as components of machines which are exposed to high mechanical, thermal and/or chemical loads during operation. For example, blades of gas turbines, in particular including those of rotors for aircraft engines, but also those for stationary gas turbines, are produced from monocrystals.
The fabrication of such monocrystalline workpieces takes place for example by directional solidification from the melt. This involves casting methods in which the liquid metal alloy solidifies to form the monocrystalline structure, i.e. to form the monocrystalline workpiece, or directed.
Known for example is a special casting method for producing such workpieces in which the liquid alloy located in a ceramic mold is given a crystallographic orientation in a directional temperature field, for example of a Bridgeman furnace. In this case, dendritic crystals are oriented along the heat flux and form either a columnar grain structure (i.e. grains which extend over the length of the workpiece and are referred to here, in accordance with the terminology generally used, as directionally solidified) or a monocrystalline structure, i.e. the entire workpiece comprises a single crystal.
In these methods, the transition to globulitic (polycrystalline) solidification must be avoided, since this non-directional growth necessarily forms transverse and longitudinal grain boundaries, which nullify the good properties of the directionally solidified or monocrystalline component. When reference is made in the present document to a monocrystalline structure or to monocrystalline structures, this both monocrystals which have no grain boundaries and also crystal structures, which indeed have grain boundaries which extend longitudinally but no grain boundaries which extend in the transversal direction. These second-mentioned crystalline structures are also referred to as directionally solidified structures.
When reference is made generally to directionally solidified structures, this means both monocrystals, which have no grain boundaries or at most small-angle grain boundaries, and columnar structures, which indeed have grain boundaries extending in the longitudinal direction but no transversal grain boundaries.
Among the alloys used for example for the monocrystal turbine blades mentioned are so-called superalloys on a nickel (Ni), cobalt (Co) or iron (Fe) basis. Nickel-based superalloys in particular have outstanding mechanical and chemical high-temperature properties.
Such components become worn and damaged during use, but can be regenerated by removing the affected areas, if necessary, and reapplying the material in these areas (for example epitaxially). However, when doing so, it is intended that the same crystal structure is achieved.
Such a method is shown in U.S. Pat. No. 6,024,792 and in EP 0 892 090 A1.
In the case of this method, a layer of the material to be applied is applied transversely in relation to the length of the surface to be treated, in each case in narrow adjacent strips which correspond the area of the surface to be treated. This results in overlapping or touching of individual weld beads (strips on which new material adjacently grows), which because of the geometrical conditions lead to non-epitaxial growth with insufficient crystal orientation.
As a consequence, mechanical properties are inadequate.